I wish to run about 50 Joomla or WordPress low-traffic websites on 1 server, or part of a server. Each website will be managed by its own, naive owner who will have be able to access the Joomla or Wordpress backend of the website. I am concerned about security and isolation as my users will periodically get into trouble by not protecting their sites properly.

Two alternatives I know of exist:

Run one Linux system with multiple websites under Apache. Follow current Joomla and WordPress security tips. Increase the isolation of the individual sites by using mpm-itk, which will allow each website to run as its own user.

The alternative to this is to run virtualization software such as the Xen hypervisor. Each site would have its own, virtual Linux system.

I lack the experience needed to make this decision and I am asking which path to take. Obviously, there may be other alternatives that I haven't considered.